A fixed speed difference and viscosity isolate the gap denominator in viscous stress. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A two-metre gap makes a six-per-second shear rate
Keep viscosity at 4 Pa*s and speed difference at 12 m/s. Gap 2 metres gives shear rate 6 reciprocal seconds and stress 24 pascals.
4⋅12/2=24
A three-metre gap lowers stress to sixteen pascals
The same speed difference across gap 3 metres gives shear rate 4 reciprocal seconds and stress 16 pascals.
4⋅12/3=16
A four-metre gap lowers stress to twelve pascals
At gap 4 metres, the rate is 3 reciprocal seconds and stress is 12 pascals. These are uniform toy shears, not velocity profiles.